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Astronomers reveal the first detailed maps of galaxy distribution in the early universe

17.07.2003

Peering back in time more than 7 billion years, a team of astronomers using a powerful new spectrograph at the W. M. Keck Observatory in Hawaii has obtained the first maps showing the distribution of galaxies in the early universe. The maps show the clustering of galaxies into a variety of large-scale structures, including long filaments, empty voids, and dense groups and clusters.

These maps are among the first results from the DEEP2 Redshift Survey, an ongoing three-year project designed to study galaxies in the distant universe over a volume comparable to recent surveys of the local universe. Using the new DEIMOS (Deep Extragalactic Imaging Multi-Object Spectrograph) instrument at the 10-meter Keck II Telescope, this project is measuring the properties of distant galaxies as well as mapping out their distribution in space. DEIMOS, which was built precisely for this survey, allows simultaneous, detailed observations of up to 150 galaxies at a time. By studying galaxies whose light has taken billions of years to reach the Earth, the astronomers are effectively looking far back in time.

"For the first time, we are getting a map of the universe as it was 7 billion years ago, when it was roughly half the age it is now. Comparing these observations with local surveys will yield direct clues to some of the most profound mysteries of the universe, such as the nature of dark matter, the nature of dark energy, and the origins of galaxies and quasars," said David Koo, professor of astronomy and astrophysics at the University of California, Santa Cruz.

Koo is presenting the groups first findings Thursday at a meeting of the International Astronomical Union (IAU) in Sydney, Australia. The DEEP2 survey is a collaboration between astronomers at UC Santa Cruz, UC Berkeley, and several other institutions, including the California Institute of Technology and University of Hawaii. Marc Davis, professor of astronomy at UC Berkeley, is principal investigator for the current phase of the DEEP2 project. Sandra Faber, University Professor of astronomy and astrophysics at UCSC, led the team that designed and built DEIMOS at the Lick Observatory laboratories at UCSC.

"The DEIMOS data are so extensive compared to previous results that we can construct maps showing considerable large-scale structure, and we can see differences in the clustering of different types of galaxies," Davis said.

The structures seen in these maps of distant galaxies are similar to structures seen in the local universe, although DEEP2 shows them at an earlier stage of development.

"We are watching the galaxy population grow into what it is today," said Alison Coil, a graduate student at UC Berkeley who was the lead researcher on the analysis of the maps.

The maps presented at the IAU meeting are the first detailed three-dimensional pictures of the spatial distribution of galaxies in the early universe. They show the locations of galaxies in the distant universe in three of the four DEEP2 fields. Although the maps do not cover much area of the sky (observations so far cover an area about four times the size of the full moon), they extend very far along the line of sight, probing deep into the universe to reveal objects that are very distant in both space and time. This is a different strategy from current local surveys such as the Sloan Digital Sky Survey and the 2dF Survey, which cover very wide areas of the sky but probe much shorter distances along the line of sight, Coil said.

"This survey is extremely deep. Not only are we seeing objects more than 7 billion light-years away, but the survey itself covers 2.5 billion light-years, so we can see the evolution of the galaxy population within the DEEP2 survey, in addition to comparing it with local surveys," she said.

Statistical tests indicate that the clustering seen in the DEEP2 maps is not as strong as that seen in the local universe today, Coil said. Galaxies cluster under the force of gravity, and this clustering grows with time as structures get larger and exert stronger gravitational pulls, causing them to grow even more. Regions of space with high densities will exert their gravitational influence on surrounding galaxies, pulling them in toward the dense regions, which become even more dense in the process. While these dense regions are growing, empty voids are left behind where very few, if any, galaxies reside.

At the same time, the galaxies themselves are evolving: creating stars, forming black holes at their centers, falling in toward each other, and often colliding and merging to form larger galaxies. All of these processes can be seen in the DEEP2 data, Coil said.

DEEP2 observations will eventually map the locations of some 60,000 galaxies at distances of about 6 to 8 billion light-years. Seeing how the large-scale structures made up of galaxies evolved over time is just one aspect of the study. The researchers are also investigating the properties of the individual galaxies: their ages, how bright they are, how much matter they contain, how quickly they are forming stars, and so on.

"Our ability to, in effect, weigh these distant galaxies is a unique aspect of our survey," Koo said. "Galaxy mass is a fundamental parameter for understanding the formation and evolution of galaxies, and by having this dimension we can study these questions with more precision than has been possible before."

Galaxies are represented on the maps with black or red symbols. The black points are galaxies that appear to be forming new stars in abundance, while the red points are galaxies that have little ongoing star formation, which generally are also older and redder. These older galaxies are much more strongly clustered in space than the star-forming galaxies. The researchers also found that galaxies with red colors are more clustered than bluer galaxies, and brighter galaxies cluster more strongly than fainter galaxies.

"These trends are similar to those seen in the local universe; but for the first time we can see that they were already in place 7 billion years after the Big Bang, before our own Sun and solar system had formed," Davis said.

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